There is no templet letter that would cover every eventuality.
Clearly this housebuilder is so disorganised they are not even able to plan a relative accurate completion date. There may well be a specific reason for this, it could be problems with utilities, but also it could be problems with the ground, the house itself or a planning issue.
Reserving a plot in April with a completion date in September is six months for the build, more than ample in most cases. What you have just been told equates to 10 months to build a house, unheard of in the industry!
There is a reason and you have a right to know. Get your solicitor to write the letter, it could be the best £100 you ever spend!